Constitutively active Bcr-Abl tyrosine kinase (TK) promotes cell growth and inhibits apoptosis in Bcr-Abl positive acute leukemia cells by activating a variety of downstream molecular mechanisms. Gleevec (previously known as CGP57148B or STI-571) is a relatively specific Bcr-Abl TK inhibitor which is highly active in CML. Recent studies from our laboratory have shown that at clinically achievable concentrations of 0.25 to 1.0 uM, Gleevec induces differentiation and apoptosis and sensitizes Bcr-Abl positive cultured human acute leukemia HL-60/Bcr-Abl and K562 cells to antileukemic drugs and Apo-2L/TRAIL. Gleevec inhibits tyrosine phosphorylation of Bcr-Abl, as well as downstream of Akt kinase and STAT5. This results in the inhibition of STAT5 and NFKB activities and lowering of the levels and activities of the antiapoptotic Bcl-xL and IAP (inhibitors of apoptosis) proteins. However, these studies need to be extended to patient-derived, CD34+ CML-blast crisis (BC) and Bcr-Abl positive ALL progenitor cells. Although Gleevec also has significant clinical activity in CML-BC and Bcr-Abl positive ALL, the complete response rates have been low and duration of most remissions short. To study the intracellular mechanism of Gleevec-resistance, we have isolated HL-60/Bcr-Abl and K562 cells that are capable of growth in the continuous presence of clinically relevant levels of Gleevec. Notably, these cells are sensitive to novel anti-Bcr-Abl targeted agents 17-allyl-amino geldanamycin (17-AAG) or PD 180970. However, the mechanisms of resistance and the activities of these novel agents that may overcome Gleevec-resistance in CD34+ CML-blast crisis (BC) and Bcr-Abl positive ALL progenitor cells need to be determined. Therefore the specific aims of this proposal are: AIM 1: To determine the molecular mechanisms underlying the antiapoptotic activity and chemoresistance due to Bcr-Abl and its inhibition by Gleevec in patient-derived, Bcr-Abl positive leukemic blast progenitor cells. AIM 2: To determine the growth inhibitory and apoptotic effects of combinations of Gleevec with novel agents, e.g., Apo-2L/TRAIL, PD180970 or 17-AAG against Bcr-Abl positive leukemic blast versus normal progenitor cells. AIM 3: To determine the Bcr-Abl based mechanisms of Gleevec-resistance in leukemic blast progenitors from patients with Gleevec-refractory CML-BC or Bcr-Abl positive adult ALL. AIM 4: To determine the in vitro cytotoxic effect, and its underlying mechanisms, of 17-AAG or PD180970 against Gleevec-resistant leukemic blast progenitor cells. These in vitro studies may help define novel therapeutic strategies for clinical testing against Bcr-Abl positive acute leukemias.